The invention relates to a current measuring system, more particularly for use as an earth leakage detection system, comprising a weak magnetic circuit having one or more primary windings to conduct the current to be measured or currents to be detected for their sum of difference value and having a secondary winding included in a series connection of an electric supply source, a switching device for changing the polarity of the voltage applied by the supply source to the secondary winding, a current detection device for supplying a switching signal to the switching device when the current passing through the winding has attained a predetermined limit value, and a load impedance integrating the current passing through the series connected circuit. Such a system is disclosed in Swain U.S. Pat. No. 3,768,011, issued Oct. 23, 1973.
As is disclosed in lines 6-12 of column 6 of U.S. Pat. No. 3,768,011, such a system exhibits a positive feedback effect. This effect results in that any asymmetry phenomena occurring in the system are amplified to an extent such that the value of the load impedance used in the system has to be limited, which may give rise to an undesirable limitation of the available output voltage. Such phenomena of asymmetry may occur in particular when as a result of an excessively slow commutation or change of polarity of the voltage supplied by the source to the secondary winding the parameters of the circuit elements, usually transistors, tend to play too important part in said commutation.